A large amount of cargo is transported by rail. Some types of cargo may be transported in bulk by simply dumping, pouring, or otherwise transferring that cargo directly into an appropriate type of rolling stock. The bulk cargo then assumes the shape of the interior of the rolling stock, thereby generally securing the cargo against movement within the rolling stock. For example, grains, other types of agricultural products, minerals, and other goods may be loaded directly into rail hopper cars. As another example, many types of liquids can be pumped directly into rail tank cars. In many other cases, however, cargo may take the form of discrete cargo units that do not necessarily assume the shape of a railcar interior. Examples of such cargo units include, without limitation, boxes, crates, drums, reinforced bags, plastic wrapped bundles, cased goods, metal coils, specialty heavy paper rolls, plastic or metal containers mounted on pallets, other types of palletized cargo, etc.
Applicable regulations require that cargo units be restrained against lateral and longitudinal shifting during rail transport. In particular, a railcar may experience significant acceleration in the longitudinal direction between the front and rear of the railcar. Such acceleration may occur as a train is repeatedly humped while additional cars are added, when the train begins moving, when the train comes to a stop, etc. If cargo units within the railcar are not restrained, they may shift and collide with each other or with walls of the railcar. The amount of cargo within a single railcar can be substantial (e.g., up to 100 tons). At a minimum, shifting cargo units can thus cause cargo damage. In some cases, damage to the railcar, derailment or other property damage can result.
Conventionally, cargo units have been restrained within railcars using dunnage materials formed from paper. These dunnage materials typically have a honeycomb or other type of cellular structure and are used to fill spaces between cargo units and/or between cargo units and interior walls of a railcar. Using such materials for cargo restraint in railcars has proved unsatisfactory in several respects. Cellular paper dunnage material is crushed when subjected to sufficient impact, but does not rebound. Although that dunnage material may protect cargo from damage during the event that causes the dunnage material to be crushed, a void space then remains around the cargo. When the next acceleration event occurs, the cargo may shift and cause damage. Furthermore, the strength of these paper-based products can be severely compromised when exposed to moisture, which can be common in an industrial shipping environment.
Another conventional restraining technique involves the use of inflatable bags placed between cargo units and/or placed between cargo units and interior walls of a railcar. However, such bags often inflate in a cylindrical manner. As a result, significant void spaces may still remain between cargo units and/or between cargo units and walls of a railcar, and cargo may not be restrained in a secure manner. Furthermore, the internal pressure in an air dunnage bag fluctuates as temperature, barometric pressure, and altitude fluctuate. If an air dunnage bag is inflated in a railcar loaded at sea level, for example, that same bag may not maintain correct pressure to perform as desired at a final destination located in a mountainous region. Because railcars commonly move across the country, such bag pressure variation can severely impact the practicality and consistency of cargo restraint methods that rely on inflatable dunnage.